Agree at the same time to withhold rent (6)
I believe the answer is:
concur
'agree' is the definition.
(I know that agree can be written as concur)
'at the same time to withhold rent' is the wordplay.
I cannot quite see how this works, but
'at' could be 'on' (location) and 'on' is found within the answer.
This explanation may well be incorrect...
(Other definitions for concur that I've seen before include "Agree, assent" , "Agree or happen together" , "meet" , "run together" , "Express agreement" .)